The big picture: The environment influences populations




  • Review: What is a population?


  • Biotic and abiotic factors affect population dynamics
    • abundance, dispersion & age structure


  • Population growth can be ‘modeled’
    • births & deaths


  • Density matters

Populations often described by their boundaries

Population density is dynamic



  • May be possible to count all individuals
    • large mammals, animals in small habitats


  • More often impractical or impossible to count all individuals
    • indirect estimates are needed
    • dusk deer surverys in Biol 303


  • Density is not static
    • immigration:
    • emigration:
    • births & deaths
    • motility

Local densities vary within a population

Population statistics change over time (demography)




  • Cohorts:
    • how many of one age-group survive to the next?
    • when does reproduction occur?


  • Girl power!
    • only females produce offspring
    • how many females give rise to new females?


  • Reproductive rates vs Survirorship

Patterns of survivorship in natural populations

Populations grow fast when resources are abundant

Can we predict changes in population size?


  • Start with ideal conditions:
    • few individuals to start
    • unlimited environment


  • Population ↑ with births & immigration


  • Population ↓ with deaths & emigration


  • Balance of births & deaths at any given time
    • per capita change (r) =

Elephants in Kruger National Park (Exponential Growth)

Resources rarely remain unlimited



  • Carrying capacity (K):
    • as population increases, resources diminish


  • Resources that limit are vast
    • food, shelter, safety, nesting sites


  • Resource limitation causes birth rate to decline


  • Logistic growth =

Trade offs exists between survival and reproduction




  • Frequency of reproduction
  • Number of offspring
  • Investment into parental care


Combined there traits represents an organism’s life history

  1. Age at first reproduction
  2. How often reproduction occurs
  3. How many offspring are created

Trade offs exists between reproduction and survival




  • Frequency of reproduction
  • Number of offspring
  • Investment into parental care


Combined there traits represents an organism’s life history

  1. Age at first reproduction
  2. How often reproduction occurs
  3. How many offspring are created

Trade offs exists between survival and reproduction




  • Frequency of reproduction
  • Number of offspring
  • Investment into parental care


Combined there traits represents an organism’s life history

  1. Age at first reproduction
  2. How often reproduction occurs
  3. How many offspring are created

Trade-offs and Life Histories

Density in populations matter




  • Why do populations stop growing?


  • Density dependent =
    • resources, predation
    • disease, toxic waste


  • Population density works as a negative feedback

Brainstorm: What factors are density independent?

Patterns in human population

Patterns in human population: Age structure

Patterns in human population: Density and carrying capacity